Austenitic heat resisting steel

ABSTRACT

Cr-Ni system austenitic heat resisting steel having higher heat resistibility at elevated temperatures and for a longer time than common austenitic stainless steel by the multiplied effects of 0.03 to 0.30 C, Ti + Nb content and that the atomic ratio of Ti + Nb/C is within range of 0.05 to 1.0, 0.2 to 4.0% Mo and 0.04 to 0.15% N.

United States Patent Mimino et a1.

Feb. 13,1973

AUSTENITIC HEAT RES'ISTING STEEL lnventorsi Tohru Mimino, No.

Assign ee Filed:

Appl. No.:

20-17, 1- chome, Minami, Meguro-ku, Tokyo; Kazuhisa Kinoshita, Totsuka-ku, Yokohama-shi, Kanagawa-ken; Ryohei Tanaka, Hodogaya-ku, Yokohama-shi, Kanagawa-ken; Takayuki Shinoda, 'Toshima-ku, Tokyo, all of Japan Nippon Kokan Kabushiki Kaisha, Tokyo, Japan March 10, 1970 US. 75/12 8 G, 75/128 N, 75/128 T,

s-ratss /mm Field of Search.75/128 N, 128 T, 128 G, 128 W [56] References Cited UNlTED STATES PATENTS 3,337,331 8/1967 Ljunberg ..75/128 T 2,159,724 5/1939 Franks ....75/128 G 2,801,916 8/1957 Harris ..75/128 G 3,303,023 2/1957 Dulis ..75/128 T 2,602,737 8/1952 Binder ....75/128 N 2,229,065 1/1941 Franks... ....75/l28 N 3,523,788 8/1970 Bates ..75/128 N Primary Examinerl-lyland Bizot Att0rney-Linton & Linton ABSTRACT Cr-Ni system austenitic heat resisting steel having higher heat resistibility at elevated temperatures and for a longer time than common austenitic stainless steel by the multiplied effects of 0.03 to 0.30 C, Ti -1- Nb content and that the atomic ratio of Ti Nb/C is within range of 0.05 to 1.0, 0.2 to 40% Mo and 0.04 to 0.15% N.

2 Claims, 4 Drawing Figures RUPTURE TIME PATENTED FEB 1 3 I973 SHEET '4 OF 4 wslF uza wa v &

n A n m MA w 00 N NNKH R [195 n A T T MMT A S m v 4. NUMA.

u HZOK H ORYA s TKRT AUSTENITIC HEAT RESISTING STEEL This invention relatives to a composition for improving the heat resistibility of Cr-Ni system austenitic stainless steel for elevated temperature services.

' It is well known that higher strength and oxidation resistibility at elevated temperatures of steel are required in some services such as boiler industries. As boilers become larger and of the ultra-critical pressure type, a higher strength of the steels employed is required.

At present .118 (Japanese Industrial Standards) SUS27, SUS29, SUS-32 and the like (AISI-304, 321, 316 and the like as similar standards) among l8Cr-8Ni austenitic stainless steels are generally employed for elevated temperatures and high pressure services.

The fact is that said 18-8 Ti-Nb steel or SUS32 steel (AISI-316 steel) is commonly employed in consynergestic) effect of Ti and Nb.

Additional objects of this invention will become apparent by the following description referring to the examples and the accompanying drawings in which:

FIG. 1 is a graph showing creep rupture strength of l8Cr-8NiTi-Nb steel, which is known;

FIG. 2 is a graph showing creep rupture strength of SUS-32 steel which is AISI-316 steel as similar Standard;

FIG. 3 is a graph showing creep rupture strength affected by added Mo contents; and

FIG. 4 is a graph showing creep rupture strength of the present invention steel.

According to the present invention, the steel has the following composition; that is 0.03 to 0.30% by weight C up to 1.0% weight Si sideration of having superior strength at high temperaup to 2.0% weight Mn tures in spite of a fault. The following table shows the 15.0 to 26.0% weight Cr chemical compositions of these steels. 7.0 to 22.0% weight Ni Percent 0 Si M11 P 0 Cr N1 M0 Ti Nb N 0.10. 0.58 1. 47 0. 01s 0. 024 18.29 0. s0 Trace 0.09 0 10 0. 0130 0. 0s 0.40 1.50 0.031 0.000 17.04 12.50 2. 47 0. 0357 0.10 0.75 1.50 0. 028 0.000 10.05 12. 44 2. a0 0. 0300 00s 32 (a) 0.00 0.58 1.05 0.020 0. 014 17.00 13.00 2. 44 0. 0225 The above 18-8 Ti-Nb steel is subject to solution up to 4.0% weight Mo heat-treatment, which is water toughened at l,l00C, 0.04 to 0.15% weight Na and 3115-32 Steel which is Wale! By weight Ti+Nb (containing Ta) contents and that gh n a l. l. and p the atomic ratio ofTi+NblC is 0.05 to 1.0 tively. The results of creep rupture test for these steels 35 A d a part or ll f th above Ti i o ible t be with are shown in FIG. 1 and FIG. 2.

Referring now to FIG. 1 and FIG. 2, it is found that the creep rupture strength of 18-8 Ti-Nb steel is far higher than that of SUS-32 steels, which is marked for long test time on purpose, owing to the following reason, that is, the coexistance of Ti and Nb (containing Ta) in 18 Cr8NI-Ti-Nb steel are effective for causing the coalescence of formed carbides, such as M C to stop and for causing said carbides to disperse uniformly, which results in improving said high temperature and long service strength, while the coalescense of said carbides of SUS-32 steel (AlSI-3 16 steel) is made to accelerate owing to M0 when being exposed at high temperatures for long time services and consequently the strength of said steel is lowered.

The present invention has been developed in order to improve the qualities of said 18 CR-SNi-Ti-Nb steel and strength of said SUS-32 steel for long time services at high temperatures. That is, the heat resisting steel of this invention is characterized by contents of Ti+Nb (containing Ta) and that the atomic ratio of Ti+Nb/C is 0.05 to 1.0 in coexistence with up to 4.0% Mo and 0.04 to 0.15%N.

An object of the present invention is to provide an economical heat resisting steel having higher strength and heat resistibility for elevated temperature services than those of SUS-32 steel (AISI-3l6 steel as similar standard).

Another object of this strength is to provide a heat resisting steel having higher strength and heat resistibility for elevated temperature services than these of l8Cr-8Ni-Ti-Nb steel, which is based on multiplied (or As mentioned above, the features of this invention consist in that Mo and N is made to coexist with said Ti+Nb. In this case, effects of added Mo to 18Cr-8Ni- The creep rupture strength of these steels, which is water toughened at l,l00C, at 650C X hr and 700C X 1,000 hr is shown in FIG. 3 in comparison with that of SUS32 steel (AISI-3 16 steel).

Referring now to FIG. 3, it will be understood that the curve of the creep rupture strength at 700C X 1,000 hr tends toward the uppermost limit by the addition of about 1%Mo, while that at 650C X 100 hr is on the rise, nearly straight. I

It is, however, evident that the creep rupture strength of l8Cr-8Ni-Ti-Nb steel is still superior to that of SUS-32 (AISI-3l6) steel to which M0 is similarly TABLE 111 (Chemical composition) Percent s1 MN P s Cr N1 M0 Tl Nb The creep rupture strength of these steels which is heat Up to 0.04% N is the common content in the usual treated in the same manner as the above steels ofTable l0 steel-making process. It is, however, difficult to im- II, at 700C is shown in FIG. 4 in comparison with that of SUS-32 steel (AISl-3 16 steel). 1

Referring now to FIG. 4, the slope degree of the creep rupture strength for long time services is far gentler than that of said SUS-32 steel. This fact signifies the effect of the added N at elevated temperature and for long time services. It is needless to say that the required contents of N should be specially added in the steel-making stage through same addition agent, e.g. Mn-Nitride. Thus, the addition effect of Mo and N has not been discovered before.

The range of the chemical composition as mentioned above is determined by the following reasons.

C content of the present invention is chosen in relation to Ti+Nb content which is within the range of 0.03 to 0.30% More than 0.30%C is undesirable, because solution C in steel precipitates as Cr C during elevated temperature services and then coalesces and results in the deterioration of the creep rupture strength. It is, conversely, difficult to improve said strength when C content is less than 0.03.

Ti+NB (containing Ta) content is determined in relation to the above C content. That is, the atomic ratio of TiNb/C should be within the range of 0.05 to 1.0. Ti+Nb content exceeding the above ratio has no effect in improving the high temperature strength. In this invention, part or all of said Ti can be replaced with Zr.

It is confirmed by experiments that Mo can improve said strength in relation to N content even though the content of Mo was small. However, less than 0.02% Mo has little effect to improve said strength. More than 4.0% Mo has a tendency to form ferrite and is costly.

prove said strength even though the Mo content was increased. Accordingly, more than 0.04% N- is required. On the other hand, more than 0.15% N will be impossible to be realized.

Less than 15.0% Cr makes said oxidation resistibility worse. When the Cr content is more than 26.0% a stable austenitic steel is difficult to be obtained. Ni is necessary to make a stable austenitic phase form. The content should be chosen within the range of 7.0 to 22.0% in relation to the Cr content.

It is, in the present invention, free from that some content of Si, Mn, or Al, which is added on purpose of deoxidation, is included in steel.

Thus, according to this invention, Cr-Ni system austenitic heat resisting steel for use at elevated temperatures and for long time services may be remarkably improved with a lower cost, and it is possible to be broadly employed in many fields of industrial circles I claim:

1. An austenitic heat-resisting steel essentially consisting of,

0.03 to 0.03% by weight C,

15.0 to 26.0% by weight Cr,

7.0 to 22.0% by weight Ni,

0.2 to 4.0% by weight Mo,

0.04 to 0.15% by weight N and Ti+Nb (containing Ta) content that the atomic ratio of Ti+Nb/C is within range of 0.05 to 1.0 and balance Fe.

2. The austenitic heat-resisting steel as set forth in claim 1 wherein at least part of said Ti is replaced with Zr. 

1. An austenitic heat-resisting steel essentially consisting of, 0.03 to 0.03% by weight C, 15.0 to 26.0% by weight Cr, 7.0 to 22.0% by weight Ni, 0.2 to 4.0% by weight Mo, 0.04 to 0.15% by weight N and Ti+Nb (containing Ta) content that the atomic ratio of Ti+Nb/C is within range of 0.05 to 1.0 and balance Fe. 